This invention relates to apparatus and methods for making semiconducting and photoelectronic devices and in general to a method and apparatus for coating a substrate with a semiconducing material. More specifically, the invention relates to the preparation of amorphous semiconductor films such as amorphous silicon.
Semiconductor films are of some significance today for use in a variety of industrial applications and are of particular interest for large area electronic applications such as solar cells, xerographic photoreceptors, thin film transistors, vidicons, etc. This interest is particularly high with respect to hydrogenated amorphous silicon films for several reasons. They have excellent photoelectric properties, high absorption coefficients through the visible region and are relatively low in cost. In addition, they can be manufactured in relatively large areas are non toxic, relatively hard and do not crystallize. Furthermore, they are ambipolar and can be xerographically charged either positively or negatively. They can also be doped to p or n type semiconductor to transport either positive or negative charge. In addition, they can be alloyed with other elements to provide material with tunable optical properties and which in particular are sensitive in the infrared region. With these properties they hold particular promise in the xerographic photoreceptor area as a potential replacement for selenium and selenium alloys. This is particularly true since they have the above superior properties and in addition are no more brittle than selenium.
This activity has been kindled by the discovery that the dangling bond defects intrinsic in the preparation of amorphous silicon can be reduced and to some extent controlled by choice of film deposition conditions. In pure amorphous silicon these dangling bonds are present in a density high enough to make films of this material unsuitable for many semiconductor and photoelectronic applications. The material, for example, cannot be successfully doped and made into p-n operation devices. These intrinsic dangling bonds furthermore serve as recombination sites making such films photoelectronically useless. In addition, the presence of impurities which introduce states in the band gap also alters the electrical properties of the film. This is particularly noticeable with metallic impurities in the film which give rise to electrically active impurity states in the band gap. If these states are present in high density it becomes impossible to control the conductivity of these materials thereby making n or p type doping not feasible. It has been found that if the amorphous films are formed in certain ways with the presence of a reactive gas that the gas coordinates with he intrinsic dangling bond defects and thereby removes these localized states from the band gap. This is particularly true for silicon which has been found to be especially reactive with hydrogen gas. The two techniques that have been used in the prior art for this general procedure are glow discharge decomposition of silane and r.f. (radio frequency) or direct current sputtering with a reactive gas.
In the glow discharge chemical vapor deposition technique silane gas (Si H.sub.4) is flowed between two electrodes, one of which has the substrate mounted on it. As power is applied to the substrate the silane is decomposed into reactive silicon hydrogen species which deposit as a solid film on both electrodes. The presence of hydrogen is important since it may coordinate with the dangling bonds in the silicon in part as the mono, di and trihydrides, and thereby serves to passivate the dangling bonds.
The r.f. or d.c. (direct current) sputtering technique the substrate is fastened to one of two electrodes and a target of silicon is placed on the other electrode. Both electrodes are connected to a high voltage power supply. A gas which may for example be a mixture of argon and hydrogen is introduced between the electrodes to provide a medium in which a glow discharge can be initiated and maintained. The glow discharge provides ions which strike the target and remove by momentum transfer mainly neutral target atoms which condense as a thin film on the substrate electrode. The glow discharge also serves to activate the hydrogen causing it to react with the silicon and be incorporated in the deposited silicon film. The hydrogen coordinates with the dangling bonds in the silicon to form mono, di and trihydrides.
Difficulties are encountered with both of these prior art techniques principally because control over the several process parameters and thereby of the plasma energy involved cannot be achieved in either of the techniques. Particularly difficult is the reproduceability of accurate control of the discharge plasma since the floating potential of the discharge plasma cannot be readily accurately measured or controlled. Further glow discharge processes are complex and depend critically on a large number of process parameters. A glow discharge consists of a multiplicity of reactants, ions, free radicals, electrons and metastable excited species. These reactants may interact via a multiplicity of reaction pathways which may be initiated or propogated in either the gas phase or growing thin film surface. Furthermore, the surface of the reactor walls and the floating potential of the plasma exerts a well known but little understood influence on the plasma chemistry. These factors are moreover strongly affected by a number of critical deposition parameters including the r.f. power and frequency, flow rate, substrate temperature, pressure, concentration ratio of gases, gas flow pattern and reactor geometry. The large number of critical process parameters and their complex interrelationships makes these processes difficult to understand and control.
In both glow discharge or chemical vapor deposition and r-f-sputtering the film is formed in the presence of either the plasma or the sputtering process thereby increasing the probability of the plasma or the sputtering introducing defects in the film being formed since both the plasma and sputtering process are inherently high energy destructive processes. These process induced defects would arise from the bombardment of the thin film by the energetic species and radiation produced in the glow discharge including excited and ionized molecules and fragments, secondary electrons and photons. Furthermore, in both the glow discharge and r-f sputtering process the dangling bond sites of silicon, for example, may be coordinated with hydrogen as the mono, di and trihydride which is not desireable since the presence of the di and trihydride present in the film leads to undersirable photoelectric properties. In particular the photoconductivity is very poor. On the other hand it is desirable to have the hydrogen exist as the monohydride since it possesses excellent photoelectric properties. Furthermore, in the glow discharge processes, the possibility of microvoids being present in the film also exists which further leads to degradation of the electronic properties.